Code broke in production. How do I handle this?

My team is quite small and there is no QA and I had to test my code all by myself. The product is internally used.

I released a new feature this month that is supposed to send out certain mails based on some business logic.

While on a vacation I was told that the code has bugs and its breaking in some cases.

What do I do now? How do I face my manager who is very critical of these things and has already explicitly told me before to be careful.